Process for converting aromatic alcohols to aromatic carboxylic acids



United States PROCESS FOR CONVERTING AROMATIC ALCO- HOLS TO AROMATIC COXYLIC ACIDS Ewald Katzschmann, Witten, Germany, assignor to Imhausen Werke, G. m. b. H., Witten, Germany, a corporation of Germany No Drawing. Application May 29, 1953, Serial No. 358,533

Claims priority, application Germany July 7, 1952 4 Claims. (Cl. 260-523) This invention relates to a process for converting aromatic alcohols to aromatic carboxylic acids. It also relates to the conversion of aromatic alcohols which are obtained by some method other than the oxidation of alkylaromatic hydrocarbons to aromatic carboxylic acids. It especially relates to the conversion of methylbenzyl alcohols to toluic acids and phthalic acids.

In application Serial No. 301,595, filed July 29, 1952, a process for the production of aromatic carboxylic acids is described in which relatively diflicultly oxidizable oxygen-containing derivatives of alkylaromatic compounds are converted into more readily oxidizable functional derivatives, and the latter are oxidized in the liquid phase with elemental oxygen or oxygen-containing gases, under pressure if desired, preferably in the presence of catalysts. Thus, aromatic alcohols are converted into their esters, for example, the acetates, and these are oxidized with air.

In the above-mentioned application Serial No. 301,595, it is stated that the idea of making the oxidation of intermediate oxidation products easier by conversion into functional derivatives is applicable to the oxidation of such materials as may be considered chemically to be oxidation products of alkylaromatic hydrocarbons, although these compounds are not necessarily formed by oxidation of alkylaromatics. Substances which, from the chemical point of view, are conceived to be oxidation products of alkylaromatics and therefore suitable for this process are, for example, aromatic alcohols, such as p-methylbenzyl alcohol, m-methylbenzyl alcohol, o-methylbenzyl alcohol, and their homologs. These may be oxidized to the corresponding carboxylic or polycar boxylic acids with elemental oxygen or oxygen-containing gases only fairly slowly. However, it has been found that their functional derivatives, such as the esters, are relatively very much more rapidly oxidized if they are treated under the same conditions as the alcohols with elemental oxygen or oxygen-containing gases. Of the esters of aromatic alcohols, those formed from the lower aliphatic carboxylic acids are especially suitable, and especially those formed from the carboxylic acids which correspond to the alcohols.

The actual source of the aromatic alcohols and their esters is of no significance for the process of oxidation according to the invention. Commercially those prod nets are especially significant which can be obtained in known fashion from the products of the chloromethylation of aromatics alkylated if occasion arises. The aromatic alcohols obtainable from the chloromethylation products or the functional derivatives of these alcohols can be broken down in known fashion into the pure isomers and then oxidized; however, the mixture of iso mers can also be processed directly.

In further development of this process, it has been found that it is not necessary to convert the aromatic alcohols into the esters in one step and to oxidize the esters in a second step but that esterification and oxidation can be carried out simultaneously in one step by oxidizing the aromatic alcohol together with an oxidationresistant esterification agent, in particular, lower aliphatic or aromatic carboxylic acids or their anhydrides. The oxidation is carried out in the liquid phase with elemental oxygen or an oxygen-containing gas, at an elevated temperature, under pressure if desired, and preferably in the presence of an oxidation catalyst. Under these conditions esterification and oxidation take place simultaneously. If the aromatic alcohols in addition contain lower alkyl groups, in particular, methyl groups, attached to the ring, then these are likewise oxidized to a con siderable extent so that polycarboxylic acids are formed.

The process according to the invention is applicable to, for example, benzyl alcohol, p-methylbenzyl alcohol, m-methylbenzyl alcohol, o-methylbenzyl alcohol, 2,4- dimethylbenzyl alcohol, 2,5-dimethylbenzyl alcohol, 3,4- dimethylbenzyl alcohol, 2,3-dimethylbenzyl alcohol, and 3,5-dimethylbenzyl alcohol, as well as the analogous polynuclear compounds. The preferred esterification agent is acetic anhydride. However, the lower fatty acids, such as propionic, butyric, etc., or the fatty acid mixture resulting from the first cut in the distillation of the prod uct from the oxidation of paraffin (termed forerun fatty acids) or their anhydrides, as well as aromatic carboxylic acids, such as benzoic and toluic acids and their anhydrides, are also suitable.

Example A SOD-gram sample of p-methylbenzyl alcohol is mixed with 500 grams of acetic anhydride and 2 grams of the cobalt salt of the forerun fatty acids having from 4 to 9 carbon atoms obtained by the oxidation of parafi'in, and the mixture oxidized at C. with 1.5 liters of air per minute. The oxidation is continued for 42 hours. The oxidate weighed 1,089 grams. Acetic acid and excess acetic anhydride are distilled off, and the 560 grams of residue saponified by boiling with sodium hydroxide solution. The saponification solution is extracted with ether to give 162 grams of unsaponifiable material having an acid number of 0.3, a saponification number of 1.0, and a hydroxyl number of 448. This unsaponifiable material, then, consists of p-methylbenzyl alcohol with some di-p-methylbenzyl ether. The saponification solution is then acidified with HCl and 346 grams of an acid mixture obtained. This mixture is then esterified with excess methanol. From the cold methanol solution 63 grams of practically pure dimethyl terephthalate is filtered. The dimethyl terephthalate obtained has an acid number of 0, a saponification number of 573, and a melting point of 140.5 C. The methanol is then removed from the filtrate by distillation to give 315 grams of a mixture of methyl p-toluate with some methyl p-hydroxymethyl benzoate. The mixture has an acid number of 20, a saponification number of 370, and a hydroxyl number of 71. This mixture can readily be further oxidized to monomethyl terephthalate.

What I claim and desire to protect by Letters Patent is:

1. The process for producing an aromatic carboxylic acid which comprises mixing a benzyl alcohol with an esterification agent selected from the lower alkanoic acid anhydrides, and submitting the mixture to an oxidation treatment in the liquid phase with a gas containing elemental oxygen.

7 acetic anhydride are mixed in approximately equivalent 2. The process of claim 1, wherein the esterification References Cited in the file of this patent agent is acetic anhydride. 7 UNITED TA P E r 3. The process of claim 2, wherein the alcohol and S S AT NTS esterification agent are mixed in approximately equiva- 2245528 Loder June 1941 lent quantities a 5 2,727,919 Saunders Dec. 20, 1955 4. The process of claim 2, wherem the alcohol and V FOREIGN PATENTS quantities.

623,836 Great Britain May 24, 1949 V 

1. THE PROCESS FOR PRODUCING AN AROMATIC CARBOXYLIC ACID WHICH COMPRISES MIXING A BENZYL ALCOHOL WITH AN ESTERIFICATION AGENT SELECTED FROM THE LOWER ALKANOIC ACID ANYHDRIDES, AND SUBMITTING THE MIXTURE TO AN OXIDATION TREATMENT IN THE LIQUID PHASE WITH A GAS CONTAINING ELEMENTAL OXYGEN. 